This invention generally relates to aircraft wheels and more particularly to a unique configuration for a eutectic fusible plug device for mounting in a wheel and automatically venting overpressure in a mounted tire due to heat generated in a wheel and brake assembly upon braking of an aircraft.
Currently in the art of venting aircraft tires, it is the practice in the simplest form to provide a thermally sensitive and responsive fusible plug in a carrier member which is received within a bore in the wheel and in a location for rapid detection of heat generated in the wheel and brake assembly. The fusible plug is a particular composition of materials, i.e. a metal alloy, which when subjected to a specific temperature level changes from a normally solid form to a flowable liquid and is easily blown out of the carrier by tire overpressure. Fusible plugs of this type are generally made by raising the temperature of a virgin eutectic material above its specified melt temperature and pouring it into a bore provided for it in the carrier member. When cooled and solidified, the eutectic seemingly presents a neat configuration and a simple temperature responsive device.
The inventors of the present invention have found, however, that while the specification of the virgin eutectic material as it is received from the manufacturer is within .+-.1.degree. F. of the specified melt temperature specification, remelting of the virgin eutectic in the production of fusible devices destroys such specification to where it may be as much as .+-.10.degree. F. of the desired value. In this circumstance, one cannot be certain that the melt temperature of a particular fusible device is what it should be when installed in an aircraft wheel.
Changes in the virgin eutectic material may be attributed to the fact that the component materials which comprise the eutectic alloy are easily oxidized when heated. Also, foreign substances and/or impurities may enter the composite during the further heating and handling procedures. In addition, air bubbles and/or voids may be created in the fusible plug configuration when the melted eutectic is poured into the carrier member.
In addition to the above mentioned possible sources which may change the eutectic specification, it has been determined through microscopic studies of these type cast-in-place fusible plugs and more particularly for example a plug comprised of ASM Item N (Tin-Silver) eutectic fusible alloy, (Metals Handbook, 8th Edition, Table 2, page 864), that at temperatures near 400.degree. F. some copper molecules from a brass configured carrier member migrate towards the eutectic material and some tin molecules from a tin-silver configured eutectic alloy migrate towards the brass carrier member. This interface molecular action between the carrier and eutectic materials results in the formation of an intermetallic layer which is very hard and brittle. Further, the intermetallic layer seems to grow rapidly at sustained elevated temperatures near 400.degree. F. It is this layer which is considered instrumental in creating "leaker" fusible plugs when subjected to thermal shock resulting from the cyclic temperature excursions experienced in the aircraft environment.
It is therefore in accordance with one aspect of the present invention, an object to provide a fusible, temperature responsive device for mounting in an aircraft wheel to vent overpressure in a mounted tire due to heat generated in a wheel and brake assembly, which device is of simple construction, may be easily installed and serviced, and exhibits no more than .+-.1.degree. F. difference from a manufacturer's established melt temperature for the eutectic material used in the device.
It is in accordance with another aspect of the invention an object to provide a eutectic fusible device which eliminates the formation of any intermetallic layer and thus assures the leakproof integrity of the fusible device in normal operations.
It is in accordance with still another aspect of the invention, an object to provide a eutectic fusible device having a particular self-holding or locking taper configuration of the constituant elements which may be merely press-fit into their operational orientation without need for special tools and which device further includes seal means to assure a positive leak-proof integrity of the device in normal operations.
These and other aspects and advantages of the invention are provided in a fusible thermally responsive device for mounting in a bore provided for it in a wheel to vent an overpressure condition caused by heat generated in a wheel and brake assembly comprising in combination:
a solid rod-shaped length of a thermally responsive virgin eutectic alloy composition having a specific diameter and a portion of its length machined towards one end thereof to a diameter such as to define a locking taper portion;
a cylindrical carrier member defining a bore through its lengthwise extent and having designated head and body portions, the head portion being of a greater diameter than the body portion and said bore adapted for receiving the rod-shaped length of virgin eutectic therein in a press-fit and locking relationship by reason of a portion of the bore length being of substantially the same diameter as the constant diameter portion of the eutectic while the remaining bore length is configured to substantially the same locking taper as machined into the eutectic;
a sealing means mounted in the bore of the carrier member to effect sealing of the constant diameter portion of the eutectic; and
a sealing means mounted about the body portion of the carrier member to effect a sealing relationship between the head portion of the carrier member and the bore in the wheel;
said device being mounted in the wheel bore by a locking relationship as between it and the carrier member and the virgin eutectic is locked in the carrier member in a direction from the inside toward the outside of the wheel.